[FR] metadata improvements

1. `export_libraries` could be optional (actually the whole **module.json** could be optional).
2. **abi.json** could contain an optional _'link'_ to the binary (relative to the AAR root). This way, a conventional AAR could be reused for prefab by adding a fixed **prefab** tree. That would be as easy as building a prefab tree structure under `src/main/resources` of a library module.

Well, include files may require special effort to be in sync with the source tree, but there are some tricks that make this easy on Android Studio.